Synthesis and Pd-catalyzed coupling of1-C-stannylated glycals

From carbohydrate-derived ketene dithioacetals to 1-C-thioglycals: a synthetic and theoretical insight

A strategy for the synthesis of 1-C-substituted thioglycals was developed from cyclic carbohydrate-derived ketene dithioacetals in a four-step sequence. The corresponding thioglycals, in two carbohydrate series, were first obtained by removal of the exocyclic glycosyl sulfoxide, followed by treatment with an organolithium reagent. Various electrophilic groups were introduced onto the thioglycal double bond after deprotonation and formation of a glycosyl lithium intermediate. A computational analysis was undertaken to gain a better understanding of the experimental results obtained in the two series.

Stereoselective assembly of C-oligosaccharides via modular difunctionalization of glycals

C-oligosaccharides are found in natural products and drug molecules. Despite the considerable progress made during the last decades, modular and stereoselective synthesis of C-oligosaccharides continues to be challenging and underdeveloped compared to the synthesis technology of O-oligosaccharides. Herein, we design a distinct strategy for the stereoselective and efficient synthesis of C-oligosaccharides via palladium-catalyzed nondirected C1-H glycosylation/C2-alkenylation, cyanation, and alkynylation of 2-iodoglycals with glycosyl chloride donors while realizing the difunctionalization of 2-iodoglycals. The catalysis approach tolerates various functional groups, including derivatives of marketed drugs and natural products. Notably, the obtained C-oligosaccharides can be further transformed into various C-glycosides while fully conserving the stereochemistry. The results of density functional theory (DFT) calculations support oxidative addition mechanism of alkenyl-norbornyl-palladacycle (ANP) intermediate with α-mannofuranose chloride and the high stereoselectivity of glycosylation is due to steric hindrance.

Investigation of the Selectivity of the Palladium-Catalyzed Aroylation and Arylation of Stannyl Glycals with Aroyl Chlorides

The selectivity of the palladium-catalyzed aroylation and arylation of 1-tributylstannyl glycals with aroyl chlorides was investigated. The selectivity was controlled by the palladium catalyst, and high selectivity was achieved via ligand modification of the palladium catalyst. The reaction catalyzed by Pd(OAc)2 provided aroyl C-glycals with high selectivity, whereas the reaction catalyzed by Pd(PPh3)4 produced aryl C-glycals with diminished selectivity. The scope and limitation of the selectivity in this reaction are discussed.

Unexpected Formation of Oxetanes during the Synthesis of Dodeco-6,7-diuloses

During the synthesis of symmetrical dodeco-6,7-diuloses that are potential candidates for inhibition of glycosidases, an unanticipated epoxide-oxetane rearrangement was observed. A bicyclic sugar consisting of a glycal moiety and an anomeric esterified furanose was oxidized under epoxidation conditions (mCPBA/KF). The isolation of the pure epoxide was not possible since a rapid reversible conversion accompanied by the migration of the ester group took place and resulted in the formation of an unusual oxetane-bridged disaccharide scaffold. X-ray diffractometric structure elucidation and the suggested mechanism of the rearrangement are provided.